This disclosure relates generally to aluminum alloys and more particularly, to aluminum alloys containing magnesium, and processes for anodizing the magnesium containing aluminum alloys.
It is well known that aluminum alloys are susceptible to corrosion. For maximum corrosion resistance, it is now almost universally accepted to anodize aluminum by using a sulfuric acid solution followed by a sealing operation that typically employs a chromated solution, a nickel acetate solution, and/or by sealing the aluminum workpiece in a bath of boiling distilled water. One such anodizing process is defined by the U.S. government in MIL-A-8625 and is also commonly referred to as Type II anodizing. The Type II anodizing process applied to aluminum and aluminum alloys produces a porous oxide coating that is about 0.001 to about 0.0003 inches thick and has a typical coating weight of about 1,000 mg/ft2. The coating thickness of the oxide layer is generally a combination of both penetration into the surface of the aluminum and build-up onto the surface, in approximately a 1:1 ratio. The resulting oxide coating provides corrosion resistance, abrasion resistance, hardness, aesthetic features, and other special electrical and mechanical properties.
Type II anodizing processes are generally formed by using an electrolytic solution of sulfuric acid at about room temperature and applying a steady state direct current density of at least about 15 amperes per square foot. The process will typically run for about 30 to about 180 minutes depending on the type of aluminum alloy used.
Aluminum and aluminum alloys are generally classified with a four-digit system that is based upon the principal alloying element. For example, the 5000 series generally refers to aluminum alloys that contain magnesium as the principal alloying additive whereas the 6000 series refers to aluminum alloys that contain both magnesium and silicon as the principal alloying additives.
The amount of alloying additive present in the aluminum alloy is generally known to affect the coating quality of the anodizing process. For example, the porous oxide layer produced by anodizing aluminum in sulfuric acid is completely transparent and colorless when produced on high purity aluminum or on aluminum-magnesium alloys or aluminum-magnesium-silicon alloys based on high purity aluminum (aluminum purity greater than or equal to about 97 weight percent). However, when the aluminum is of lower purity, i.e., less than about 97 weight percent aluminum, the resulting anodized film is colored and exhibits low gloss. For example, standard Type II anodizing of a 5000 series aluminum alloy, wherein the magnesium is greater than 3 weight percent, results in a discolored coating. Typically, the discoloration will be gray in color, which is generally dependent on the amounts of alloying additive contained in the aluminum metal. The severity of the discoloration will detract from the aesthetic qualities of the anodized coating and may prevent color finishing through color anodizing techniques such as by addition of pigments or dyes, or by electrodeposition of metals to the base of the pores. Color finishing through color anodizing techniques imparts a very decorative finish both in a satin and a polished surface result.
Also, when anodizing aluminum alloys containing greater than 3% magnesium there is a reduction in surface gloss. Studies suggest that surface roughness increases during the anodizing process because magnesium reacts faster than aluminum in the sulfuric acid anodizing bath. At magnesium alloy additive levels less than 3%, the effect on surface roughness (gloss) is minimal and less pronounced. However, aluminum alloys containing magnesium alloying additive levels greater than 3%, the effect on gloss is more pronounced. Reducing the initial surface roughness of the aluminum alloy part to be anodized fails to compensate for the reduction in gloss. For example, conventionally anodizing (Type II) an aluminum alloy containing about 5% magnesium at a current density of 15 amperes per square foot that had been mechanically polished to a surface roughness less than about 100 nanometers resulted in a 20-micrometer thick oxide film exhibiting a surface roughness of about 500 nanometers. The resulting aluminum alloy oxide layer was grayish in color and exhibited low gloss characteristics.